1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to memory devices with a metal-rich metal-chalcogenide region, such as a silver-rich silver selenide layer.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), Flash ROM, and the like.
Another type of non-volatile or low-volatility memory utilizes resistance variable material cells. A programmable conductor random access memory (PCRAM) includes resistance variable material cells that utilize a controllable, stable, change in conductivity (or resistance) by the application of a voltage pulse.
Information can be stored in a PCRAM cell by forming or disrupting one (or more) conductive pathway(s), which varies the conductivity or resistance of the cell. Forming a conductive pathway programs the cell to a high conductivity (i.e., low resistance) state. Disrupting a conductive pathway programs the cell to a low conductivity (i.e., high resistance) state. In response to an applied electric field, i.e., a write voltage, which can be generated by an electric potential applied between electrodes proximate a memory element, a conductive pathway forms and the cell is written to a high conductivity state. Information can be read or retrieved from the cell by sensing the resistance of the cell using a read voltage potential of the same polarity, but less magnitude, as the write voltage. The cell can be erased, or written to a low conductivity state, by applying a voltage pulse of opposite potential and about the same magnitude as the write voltage.
Conventional fabrication processes for PCRAM technology include techniques that diffuse silver (Ag) through a silver-permeable material, such as a chalcogenide glass. One example of such a chalcogenide glass is germanium selenide (GexSe100−x). For example, a conventional process for producing a PCRAM applies silver (Ag) photodoping to a chalcogenide glass, such as germanium selenide, e.g., Ge30Se70. It is relatively difficult to diffuse silver uniformly across a wafer using conventional techniques. For example, in one conventional process, the doping of the silver varies across the wafer by about 5% to about 10%, which can reduce production yield.
Silver-rich silver selenide is relatively difficult to produce using direct deposition techniques. For example, when silver (Ag) is directly deposited on silver selenide (Ag2−δSe), where 0≦δ≦1, in an attempt to create silver-rich silver selenide (Ag2+xSe), the silver (Ag) may agglomerate on the silver selenide and create a rough surface. Similar results may occur when attempting to deposit silver selenide (Ag2Se) directly on silver (Ag). Use of a silver selenide region or layer proximate a chalcogenide glass, especially germanium selenide, can eliminate the need to photodope the chalcogenide glass with silver to achieve a functioning memory cell. Such a layer of silver selenide can contribute the necessary silver for PCRAM memory function.
What is needed is a process to enrich a material, including non-transparent materials, with a metal, such as silver (Ag), to fabricate materials such as silver-rich silver selenide (Ag2+xSe) or silver-rich silver telluride (Ag2+xTe). Such materials can be useful in cell bodies of memory devices.